1. Field of the Invention
The present invention relates to an electro-chemical device, which has a film package. More particularly, the present invention relates to an electro-chemical device capable of being processed with reflow soldering using a lead-free solder.
2. Description of the Related Art
A film package is used in various electro-chemical devices, such as an electric double-layer capacitor, a Lithium ion capacitor, a redox capacitor, and a Lithium ion battery.
The schematic structure of an electric double-layer capacitor, which is representative of these electrochemical devices, is explained. In an electric double layer capacitor in general, an electrodes part is encapsulated with an electrolyte covered by a package, which is formed of laminated films. The electrodes part is formed by stacking a set of layers of a thin polarized electrode for a positive electrode, a thin film separator, and a thin polarized electrode for a negative electrode one after another. Moreover, the electric double-layer capacitor has one terminal (a positive terminal) electrically connected to the aforementioned polarized electrode for a positive electrode, and another terminal (a negative terminal) electrically connected to a polarized electrode for a negative electrode. Both terminals are exposed to the exterior of the film package.
The electric double-layer capacitor is manufactured by the following method, for example. First, a rectangular laminated film of a predetermined size is prepared. The laminated film is formed by laminating a plastic protective layer, a metal barrier layer, and a plastic heat-seal layer one on top of the other. Next, the aforementioned electrodes part permeated with an electrolyte is laid out on the side of the heat-seal layer of the laminated film. The aforementioned laminated film is folded so that the laminated film covers the aforementioned electrodes part, and at the same time, the positive terminal and the negative terminal are exposed to the exterior of the laminated film. Then, the aforementioned film package is formed by putting together the edges of the heat seal layer at three open sides of the laminated film, and by heat-sealing the respective edges of the laminated film.
Following the trend of miniaturizing various electrochemical devices, such as the above-mentioned electric double-layer capacitor, in recent years, it is desired that electrochemical devices be mounted on a circuit board by reflow soldering using a lead-free solder in the same manner as that for general electronics parts. That is, there exists a demand for electrochemical devices capable of being processed with reflow soldering using a lead-free solder.
However, in conventional electrochemical devices having a film package, the film package is not suitable for reflow soldering using lead-free solder. In fact, in conventional electrochemical devices, the film package exhibits expansion and deformation as a result of the heat transferred to the electrochemical device in reflow soldering, (for example, by an increased vapor pressure of the electrolyte), when the devices is subject to a reflow soldering process in a reflow oven at a high temperature (at 250° C. for example). Because of the expansion and the deformation, the electrochemical devices are susceptible to various failures, such as leaking of the electrolyte or breaking of the film package.
Japanese Laid-Open Patent Application Publication No. 2002-245999 discloses a structure of an enclosed electrochemical device having a film package in a case. But, because the case does not prevent the film package from the heat expansion and the deformation, the electrochemical device is susceptible to similar or same failure modes as described above.